Add more background and links on mars data collection

Author: mashalifshin

README.md

@@ -1,27 +1,70 @@
 # mars-telemetry
 
-Contains the metrics.yaml file documenting the metrics collected by the Unified API. 
+This repo contains the yaml files specifying the metrics collected by the Mozilla Ads Routing Service (MARS).
 
-Used by https://github.com/mozilla-services/mars 
+[MARS repo](https://github.com/mozilla-services/mars )
 
-## Additional Context for Data Stewards
+[MARS Glean dictionary](https://dictionary.telemetry.mozilla.org/apps/ads_backend)
 
-* The Unified API (MARS) uses Glean for **server-side** data collection.
-* Telemetry is collected through MARS to measure ad performance and effectiveness.
-* Users cannot opt out of specific metrics – data collection is tied to ad usage. Users who disable ads entirely have no ad-related telemetry collected.
+# Background
 
-## Making Changes to Collected Data
+MARS is a backend API service that functions as a privacy-preserving proxy bewteen Firefox and third party ad providers.
+
+MARS handles requests for ads from the Firefox browser, processes them to redact or anonymize our users' information, forwards along these anonymized requests to third party ad providers, and returns privacy-respecting, tracker-free ads to Firefox.
+
+Some examples of these ads are the Sponsored Shortcuts and Sponsored Stories found on Firefox's Home and New Tab.
+
+An example of how MARS functions to preserve user privacy:
+* Instead of passing the Firefox user's potentially fingerprintable User Agent string to ad partners, MARS sends along only the user's OS and whether they are on Desktop, Tablet, or Phone. The ad partner gets enough information to return an ad appropriate for the device, but has no way to identify, fingerprint, or track any user.
+
+# Data Collection in MARS with Glean
+
+MARS is a stateless service and doesn't collect or store any data itself. The only data it persists is via Glean ping to Mozilla's data warehouse. MARS also plays a privacy-preserving proxy role between Firefox users and our own data warehouse.
+
+## Necessary data
+
+MARS needs to collect some anonymized, agreggated data about user interactions with ads in order to do honest business with our third party ad partners and advertisers, and for our own financial record keeping and auditability. This is because we are paid based on how many times we show the ads to users, and/or how many times users click on the ads we show. This is Category 2 "Interaction" data, captured via our [interaction ping](https://dictionary.telemetry.mozilla.org/apps/ads_backend/pings/interaction).
+
+MARS also collects some anonymized, agreggated data to ensure our systems are functioning correctly and our third party partners are meeting their contractual obligations. This is Category 1 "Technical" data, captured via our [request-stats](https://dictionary.telemetry.mozilla.org/apps/ads_backend/pings/request-stats) and [provider-request-stats](https://dictionary.telemetry.mozilla.org/apps/ads_backend/pings/provider-request-stats) pings.
+
+## Server-side Glean
+
+MARS's Glean integration is server-side, and all our pings are sent without any of the `client_info.*` fields that would be populated in a typical client-side Glean integration. This means no client info ever gets sent by default. Instead we explicitly and intentionally pick a few limited, non-identifying client fields to safely include for necessary reporting, and place them under [the `ad_client` category](https://dictionary.telemetry.mozilla.org/apps/ads_backend?page=1&search=ad_client.) of the ping's metrics.
+
+## Opt-out
+
+At Mozilla we always give users meaningful choice and control over data collection. To opt-out of Mozilla Ads data collection, a user must opt out of ads entirely by going to Preferences > Home and unchecking "Support Firefox", or unchecking both "Sponsored shortcuts" and "Sponsored stories".
+
+This is because we invoice our third party ad partners and advertisers based on this data. So if we are showing ads, it is necessary to keep anonymized, agreggated data about user interactions with ads, for doing honest business and for our own financial record keeping and revenue auditability.
+
+## Preventing Persistent Identifiers
+
+Firefox apps that use MARS are required to send `ContextId`, a UUID, with their ad requests. The `ContextId` is used to enable features like blocking particular ads from being shown again for the same user. The `ContextId` is one of the metrics we send in Glean pings and store under the `ad_client.` category.
+
+Clients of the MARS API are required to rotate their `ContextId`s at least every 3 days to prevent it from becoming a persistent identifier within our data warehouse.
+
+## Data Deletion
+
+MARS is currently a stateless service, we do not store any data on the users' behalf. The only data we store is the Category 1 and Category 2 data that we send in our Glean pings. This data is necessary to retain for our business purposes, so we do not provide a way for users to delete it.
+
+However, MARS does have a `/delete_user` endpoint set up, so if in the future we decided to store data on users' behalf, we have a mechanism ready that Firefox clients can use to give users the controls to delete their data.
+
+# Making Changes to Collected Data
 
 > At Mozilla, like at many other organizations, we rely on data to make product decisions. But here, unlike many other organizations, we balance our goal of collecting useful, high-quality data with our goal to give users meaningful choice and control over their own data. The Mozilla data collection program was created to ensure we achieve both goals whenever we make a change to how we collect data in our products.
 
 [*Data Collection at Mozilla*](https://wiki.mozilla.org/Data_Collection)
 
-Making changes to our data collection practices requires additional review by a Data Steward.
+Making changes to the metrics and pings in this repo requires review by a Data Steward, in addition to the usual Ads Engineering code review.
+
+## Data Steward Review
 
 1. Submit your PR to `mars-telemetry` (but do **not** merge it yet!)
 2. Fill out a [data collection review form](https://github.com/mozilla/data-review/blob/main/request.md) ([examples](https://bugzilla.mozilla.org/show_bug.cgi?id=1900898) [here](https://bugzilla.mozilla.org/show_bug.cgi?id=2006440)).
 3. Submit the request to Bugzilla.
 4. Add a comment to your PR linking your Bugzilla request and a copy of your proposed measurements table (from the data collection review form).
 5. Send a friendly message to [#data-stewardship-help](https://mozilla.enterprise.slack.com/archives/C07LMRQ5Q6B) to request a review. Make sure to give some brief context on the change, and include a link to the PR and Bugzilla request.
 
-Please note that any data collection modifications involving category 3 or 4 data will also need to follow the [Sensitive Data Collection Review Process](https://wiki.mozilla.org/Data_Collection#Step_3:_Sensitive_Data_Collection_Review_Process) outlined in the Data Collection wiki page.
+## Sensitive Data Review
+
+If the data collection changes involve Category 3 or Category 4 data, or if it is initially unclear which Category the data might fall under, then the change will also need a [Sensitive Data Collection Review](https://wiki.mozilla.org/Data_Collection#Step_3:_Sensitive_Data_Collection_Review_Process), as outlined in the Data Collection wiki page.